Most relevant response in one of the blog responses I found, albeit for XP and Vista, is below.

Still wonder if 2+1 is really giving me any edge over 2+2gb RAM or if I can just save the money and return the 2gb to the store.

Anna N28 Jul 2008 5:19 AMMy rule of the thumb, 1GB for XP on subnotebook (more RAM means higher power and shorter battery life), 2GB for every other XP, 4GB for Vista. More than 4GB only on 64bit Vista AND only if you have profiled demanding application you need to use and it needs more RAM.

Yes, there's a reason to have 4GB under 32bit OS, and that is because most graphic cards have 512MB or less, so even after drivers' memory holes you have usually much more than 2GB available, and that's useful for caching (so for casual usage, too), especially under Vista. 4GB is (on current platforms) better than 3GB because of dual channel memory so 2x2GB means better memory performance than 2GB+1GB. This may change on triple-channel Nehalem, maybe 3x1GB will be better.

Re: Usable RAM stays at 2.75gb when upgrading from 3 to 4bg

You'll need a 64-bit OS to get all 4GB.
Going from 3 to 4 isn't too significant on the X100e as you'll likely be bottlenecked by the CPU. Still, if you need the RAM, by all means. Heavy browsing will require a fair bit of RAM.

Re: Usable RAM stays at 2.75gb when upgrading from 3 to 4bg

thanks. Are you saying that my video memory is taking 1.25gb right now? If it just takes .25gb then i am wondering what happens to the remaining 1gb. can the applications or any part of the OS make good use of that? if not i'll just put back the 1gb card and return my second 2gb card to the store.